The present invention relates to digital communications, and more particularly to mobile wireless systems and methods.
Spread spectrum wireless communications utilize a radio frequency bandwidth greater than the minimum bandwidth required for the transmitted data rate, but many users may simultaneously occupy the bandwidth. Each of the users has a pseudo-random code for “spreading” information to encode it and for “despreading” (by correlation) received spread spectrum signals and recovery of information. Such multiple access typically appears under the name of code division multiple access (CDMA). The pseudo-random code may be an orthogonal (Walsh) code, a pseudo-noise (PN) code, a Gold code, or combinations (modulo-2 additions) of such codes. After despreading the received signal at the correct time instant, the user recovers the corresponding information while other users' interfering signals appear noise-like. For example, the interim standard IS-95 for such CDMA communications employs channels of 1.25 MHz bandwidth and a pseudo-random code pulse (chip) interval Tc of 0.8138 microsecond with a transmitted symbol (bit) lasting 64 chips. The wideband CDMA (WCDMA) proposal employs a 3.84 MHz bandwidth and the CDMA code length applied to each information symbol may vary from 4 chips to 256 chips. The UMTS (Universal Mobile Telecommunications System) approach UTRA (UMTS Terrestrial Radio Access) provides a spread spectrum cellular air interface with both FDD (frequency division duplex) and TDD (time division duplex) modes of operation. UTRA currently employs 10 ms duration radio frames partitioned into 15 time slots with each time slot consisting of 2560 chips (chip interval Tc=260 nanoseconds).
The CDMA code for each user is typically produced as the modulo-2 addition of a Walsh code with a pseudo-random code (two pseudo-random codes for QPSK modulation) to improve the noise-like nature of the resulting signal. A cellular system could employ IS-95 or WCDMA for the air interface between a base station and multiple mobile user stations.
A spread spectrum receiver synchronizes with the transmitter in two steps: code acquisition followed by code tracking. Code acquisition performs an initial search to bring the phase of the receiver's local code generator to within typically half a chip of the transmitter's, and code tracking maintains fine alignment of chip boundaries of the incoming and locally generated codes. Conventional code tracking utilizes a delay-lock loop (DLL) or a tau-dither loop (TDL), both of which are based on the well-known early-late gate principle.
In particular, a DLL correlates the received signal with two outputs of the local code generator: one delayed by δ and one advanced by δ where the offset δ is less than one chip interval (Tc). The result of these two correlations, together with the correlation at the perceived “on-time” instant, form the decision statistics of the time tracking unit. The time tracking method can be coherent or non-coherent. The former approach attempts to eliminate the phase rotation introduced by the channel and remove in this way the phase dependence from the decision statistics. The latter approach removes the phase dependence by squaring the result of the correlation and considering only the resulting magnitude. The latter approach is simpler, and for the cases of interest, it provides practically the same symbol-error-rate (SER) performance as the former.
The late R+(δ) and early R−(δ) correlations of the local code with the received signal are squared and subtracted. The sign of the result indicates towards which of the two instants the actual on-time instant of the received signal is closer. The value of the result indicates the degree of proximity. Since the value of the result also depends on the strength of the received signal, to remove this dependence and have the final decision statistic depend only on the time error, the result of the subtraction is divided by the square of the correlation at the perceived on-time instant. This serves to normalize the decision statistic in terms of signal power. The final decision statistic is therefore given asS=[R+(δ)2−R−(δ)2]/R(0)2and depends only on the time error and not on the received signal power or phase. S is known as the discriminator curve or S-curve (because of its shape) of the tracking loop. The absolute value of S is compared against a threshold value and, if it is larger, the on-time sampling instant is changed by one sample in the direction specified by the sign of S; while if it is smaller, no change is made in the sampling instant. The purpose of the threshold is to ensure that a time correction takes place only when the time error exceed a certain value and to help guard to a certain extent against poor quality values of the decision statistic that may be caused by noise and fading.
The TDL operates analogously but employs only a single multiply loop. The TDL dithers the clock (forward and backward delay) by δ and emulates the DLL with a comparison of advanced and delayed clock samples of the code multiplication with the received signal. Typically, DLL/TDL circuitry utilizes sampling of various signals at rates such as 8 samples per chip (or 4 samples per chip), and the offset 6 would be in the range of 1–4 samples (or 1–2 samples for a sampling rate of 4 samples per chip).
The air interface leads to multipath reception from a single transmitter, and a rake receiver has individual demodulators (fingers, tracking units) tracking separate paths and combines the finger outputs to improve signal-to-noise ratio (SNR or Eb/N0). The combining may use a method such as the maximal ratio combining (MRC) in which the individual detected signals in the fingers are synchronized and weighted according to their signal strengths or SNRs and summed to provide the decoding statistic. That is, a rake receiver typically has a number of DLL or TDL code tracking loops together with control circuitry for assigning tracking units to the strongest received paths.
However, in many applications, the time separation of multiple paths is less than or equal to the range of the discriminator characteristic of the tracking DLL/TDL. Moreover, such multipaths are nearly continuous in nature in the sense that there are frequently strong enough paths with time separation much less than one chip interval. Whenever such paths cannot be separated, they appear as a single path that exhibits fading. In many communication environments, such as urban and indoor, there is a considerable probability that sufficiently strong multipaths are separated by an amount comparable to the chip interval and hence are well within the non-zero range of the discriminator characteristic of the DLL/TDL tracking loop. The discriminator characteristic (S-curve) is typically non-zero for an advance/delay of up to 1.2–1.5 chips depending on the value of the early/late offset δ. Prominent examples of communication systems that exhibit these problems are the IS-95 and wideband CDMA cellular/PCS systems in urban and indoor environments.
Whenever two or more paths that are used for demodulation (as in a Rake receiver) are separated in time by less than the non-zero range of the discriminator characteristic of the tracking DLL/TDL, the decision statistic of the DLL/TDL for each path is affected by the presence of the other path. In other words, the shape of the S-curve itself may change and the actual results may be different than the ones theoretically expected. This can significantly degrade the performance of the conventional DLL/TDL and lead to unacceptably large time errors. Moreover, the decision statistics for the weaker path may be completely overwhelmed by the presence of the interfering, stronger path. This can lead to complete loss of synchronization for the weaker path. It is therefore highly desirable to mitigate those effects and eliminate the shortcomings of the conventional DLL/TDL time tracking methods in multipath channels. The problem of having interfering paths affecting the decision statistics of the tracking unit of a desired path was identified by A. Baier et al., “Design Study for a CDMA-Based Third Generation Mobile Radio System”, IEEE JSAC, Vol. 12, May 1994, pp. 733–743. The authors proposed tracking the envelope of each path instead of relying on the tracking unit decision statistics.
When two (or more) paths have a phase difference within one chip, then having a finger assigned to each path provides gain due to the path diversity. However, if the two paths coalesce into a single path, then two fingers assigned to the same path results in a loss due to the extra noise introduced.